I got married this past April at Dreams Las Mareas in Guanacaste. It was absolutely amazing! The reviews for the RIU weren't that great on TripAdvisor when I was researching a venue but I can't tell you first hand since I didnt visit the hotel.
The Dreams officially opened in January so its brand new and beautiful. Beach isn't white sand and there are sting rays to be careful of but we mostly stayed at the pools which are lovely. I really had the time of my life and would recommend Cinthya, my coordinator, to anyone. it is very removed and remote location but if you are looking for a more chill and relaxing time then this is for you.